Many power steering apparatus have been developed in which, based on a detected steering torque exerted on a steering wheel by the driver of a vehicle, an electric motor for steering assistance is driven to operate so that the force applied to the steering wheel by the driver is assisted by the rotational force of the motor to provide the driver with good steering feel.
When the driver of a vehicle operates the steering wheel so as to steer the vehicle to the right or left from its neutral or straight-running state, the steerable wheels are subjected to resistive forces acting thereon from the road surface in the direction to resist the steering motion of the driver. Such resistive forces become considerably great particularly when the vehicle stands still, travels at low speed, or travels on a bad road, so that the driver is required to exert a relatively large force for proper steering operation. Therefore, in order to alleviate the force or labor of a driver required for steering and enable the driver to have good or excellent steering feel, many power steering apparatus have been developed and realized. Of these power steering apparatus, there is a one of the type in which steering torque, which is generated on a steering shaft or column operatively connected with the steering wheel in accordance with steering efforts exerted thereon by the driver, is detected by a torque sensor which is installed on the steering column so that on the basis of the direction and magnitude of steering torque thus detected, a motor is operated to run thereby to provide an appropriate assistant force for assisting the steering operation of the driver.
In this type of power steering apparatus, the steering column comprises an input shaft operatively coupled to the steering wheel, an output shaft operatively coupled to a motion-conversion mechanism which converts rotational movement of the steering wheel into steering movement of the steerable wheels, and a torsion bar through which the input and output shafts are connected with each other in a coaxial relation. The steering torque exerted on the steering wheel by the driver is detected by the torque sensor such as a potentiometer provided on the junctions of the input and output shafts as a relative displacement between the input and output shafts due to a twist of the torsion bar, e.g., as an output voltage of the potentiometer. The rotational force of the motor is transmitted to an intermediate portion of the output shaft near the location at which the torque sensor is installed.
With the power steering apparatus as constructed above, due to the arrangement where the potentiometer serving as the torque sensor is mounted on the steering column which rotates together with the steering wheel, the application of an input voltage to and the taking out of an output voltage from the potentiometer are effected through slip rings, which are disposed around the potentiometer, and brushes which are secured to a housing surrounding the steering column and which are in electrical and sliding contact with the slip rings. However, such an arrangement involves the following disadvantages: in the event that the electrical contact between the brushes and the slip rings are poor, there arises the possibility that it becomes impossible to effect the detection of steering torque; and the accuracy in detection of steering torque is substantially reduced. For these reasons, it is required to make assembly of these contacting portions with great care, thus resulting in increased time and labor for assembling thereof.
On the other hand, even if such assembling has been properly effected and the electrical contact between the brushes and the slip rings is good at the initial periods, a poor contact therebetween resulting from wear over time will unavoidably take place so that the detection accuracy of the torque sensor deteriorates as time passes. Further, the electrical connection between the brushes and the slip rings are sometimes made discontinuous due to vibrations of the brushes which are caused by vibrations of the body of a vehicle during travel thereof, as a result of which it becomes impossible to detect steering torque in a continuous manner whereby the motor is driven to run discontinuously or intermittently, causing queer or unpleasant steering operation.
The above problems and disadvantages will also be involved in a power steering apparatus in which instead of a potentiometer, a strain gage, which is attached to a portion of the steering column, is employed as the torque sensor.